x/sys/windows: GetDiskFreeSpaceEx can't find any path

[scrouthtv]

What version of Go are you using (go version)?

$ go version
go version go1.16.6 windows/amd64

Does this issue reproduce with the latest release?

yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\lenni\AppData\Local\go-build
set GOENV=C:\Users\lenni\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\lenni\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\lenni\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\lenni\scoop\apps\go\current
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\lenni\scoop\apps\go\current\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.16.6
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=D:\tmp\drive\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\lenni\AppData\Local\Temp\go-build1962934238=/tmp/go-build -gno-record-gcc-switches

What did you do?

I'm trying to get the disk usage under Windows using Go. This is my code:

package main

import (
	"fmt"
	"unsafe"

	"golang.org/x/sys/windows"
)

func main() {
	p := "C:\\tmp\\"
	var caller, tot, free uint64 = 0, 0, 0

	ptr := (*uint16)(unsafe.Pointer(&p))
	err := windows.GetDiskFreeSpaceEx(ptr, &caller, &tot, &free)

	fmt.Println(err)
	fmt.Println(free, caller, tot)
}

For this to work, the folder C:\tmp\ must exist and be user-readable.

What did you expect to see?

I expected some output along the lines of

nil
1951571968 1951571968 2752507904

because that's how much free space I have on my C: drive.

What did you see instead?

The system cannot find the path specified.

The same program but in C works:

#include <fileapi.h>
#include <errhandlingapi.h>
#include <stdio.h>

int main() {
	LPCWSTR path = L"C:\\tmp\\";
	ULARGE_INTEGER caller = { 0 }, total = { 0 }, free = { 0 };

	BOOL ok = GetDiskFreeSpaceExW(path, &caller, &total, &free);

	if (ok) {
		printf("done: %s has %lu b/%lu b\n", ok ? "ok" : "error", caller, total);
	} else {
		DWORD err = GetLastError();
		printf("error: %d reading %ls\n", err, path);
	}
}

(note: the C program would fail if the folder is specified without a trailing path separator)

Thanks in advance.

[bcmills]

The functions in x/sys/windows are all the W variants (UTF-16), not the A variants (ASCII).

This line:

	ptr := (*uint16)(unsafe.Pointer(&p))

forcibly casts a pointer to a Go string header (a struct containing a pointer and length) to a *uint16, but that does not encode the string. (A Go *string is not a C LPCWSTR.)

You need to call windows.UTF16PtrFromString instead of using an unsafe.Pointer conversion.